This invention relates to a power supply and particularly to a boost converter therefor.
Power supplies for generating a predetermined voltage/current for a given application can make abnormal demands on the line supply caused by the harmonic content of the current drawn from the line. In particular, a high third harmonic content can give a large neutral current which can place unacceptable loads on the line supply transformer.
To compensate for this problem, boost-type power factor correction circuits have been developed to reduce the harmonic content. The correction circuits modify the current wave form to reduce the harmonics and thus follow more closely a sinusoidal wave form in phase with the line voltage. Such circuits generally comprise an electronic switch which is cyclically switched to control current through a boost inductor, the duty cycle of the switch being pulse width modulated so that the peak switch current, or preferably the average current in the supply return which comprises the average switch current plus the average load current, follows the line voltage while retaining the output voltage at a constant level. Control of the switch is performed by a high power factor pre-regulator circuit and a suitable circuit for performing this function is manufactured by Unitrode Integrated Circuits Corporation under their references UC1854, 2854 and 3854.
In general, power factor correction circuits are designed to work over all usual line voltages used worldwide, typically 65 VAC-265 VAC and provide a well regulated output voltage for input to a bulk convertor of standard design which performs the required voltage/current conversion for a particular application.
It is a disadvantage of such power factor correction circuits that the efficiency of the circuit decreases as the peak input voltage decreases, as the circuit has to work harder to generate the regulated output voltage to the bulk convertor, and thus greater operational losses are incurred.